Solar cells and photodetectors have not benefited from recent advances in diffractive optics. Random, wet-chemical texturing techniques to form pyramids in the (100) Silicon (Si) crystal orientation have been extensively applied for both reducing reflection and enhancing absorption by increased oblique coupling into the solar cells (See, e.g., P. Campbell and M. A. Green, Appl. Phys. Lett. 62, 243 (1987)). Applicability of these texturing processes to thin-films (≦20 μm), and low-cost, multi-crystalline (mc) Si are limited due to their large dimensions and preferential (100) crystal etching mechanisms. In recent years, random reactive ion etching (RIE) techniques have been developed for both thin films and mc-Si (See, e.g., Y. Inomata, K. Fukui, K. Shirasawa, 9th Internat. PVSEC, 109 (1996), D. S. Ruby, Saleem H. Zaidi and M. Naraynan, in 28th IEEE PVSC Conference held in Alaska in October 2000). Both of these techniques, when combined with suitable anti-reflection films, not only reduce absolute hemispherical reflection to ˜5%, but also enhance near-IR (850-1200 nm) absorption by increased oblique coupling into the surface. By contrast with random wet or dry etching techniques, periodic structures offer highly controllable mechanisms for Si reflection and absorption response over a wide spectral range. T. K. Gaylord, W. E. Baird, and M. G. Moharam in Appl. Opt. 25, 4562 (1986) have described rigorous models for rectangular-profiled grating structures exhibiting zero reflection for a suitable choice of grating parameters. D. H. Raguin and G. M. Morris in Appl. Opt. 32, 1154 (1993) have determined broadband anti-reflection properties of 1D triangular and 2D pyramidal surfaces. Ping Shen, A. N. Bloch, and R. S. Stepleman in Appl. Phys. Lett. 43, 579 (1983) have reported wavelength-selective absorption enhancement of thin-film (˜2 μm) amorphous Si solar cells by grating coupling into waveguide modes. In “Submicrometer Gratings For Solar Energy Applications” by Claus Heine and Rudolf H. Morf, Appl. Opt. 34, 2476 (1995) enhanced near-IR absorption in ˜20 μm thick Si films by diffractive coupling has been demonstrated. Broadband and narrowband spectral reflection response of subwavelength Si grating structures has been reported by Saleem H. Zaidi, An-Shyang Chu, and S. R. J. Brueck in J. Appl. Phys. 80, 6997 (1996). Enhanced near-IR response of subwavelength grating solar cells has recently been demonstrated by Saleem H. Zaidi, J. M. Gee, and D. S. Ruby in 28th IEEE PVSC Conference held in Alaska in October, 2000.
Gaylord et al., supra, describes the anti-reflection properties of 1D rectangular grating structures; however, the need to create absorption close to the solar cell junction, particularly in the near-IR spectral range is not discussed. Heine and Morf, supra, presents a diffractive approach aimed at improving solar cell response at λ˜1.0 μm. In a thin-film solar cell, near-IR absorption is weak due to the indirect bandgap of Si. By fabricating a grating structure at the back surface of the cell, enhanced absorption can be achieved by efficient coupling of the incident beam into two diffraction orders for a symmetric profile, or a single diffraction order for a blazed profile. Heine and Morf teaches away from the use of a front surface grating because of surface passivation issues. That is, Heine and Morf teaches that light-trapping that does not increase the surface area at which increased recombination losses would outweigh the benefits of increased light absorption is accomplished by effective separation of the electronic device structure in which electron-hole pairs are generated and collected from the optical structures used for enhancing the absorbed light within the electronically active region in the Si. By proper design of grating parameters, Heine and Morf teaches the choice of direction of propagation of diffraction orders such that at angles larger than the critical angle, these are trapped due to total internal reflection.
In “High Efficiency Solar Cells Enhanced With Diffraction Grating” by Sin-ichi Mizuno et al., Technical Digest of the International PVSEC-11, Sapporo, Hokkaido, Japan, 341 (1999), diffraction gratings are secured onto the front and back surfaces of thin film, single crystal silicon using an adhesive material having a refractive index of 2, such that the average light path on the surface of thin film silicon was tripled when compared with silicon thin films having a mirror placed on the back surface thereof.
Accordingly, it is an object of the present invention to improve light absorption in general, and IR absorption in particular of silicon solar cells and photodetectors by utilizing a front-surface grating structure to couple light into obliquely propagating diffraction orders inside silicon.
Another object of the present invention is to improve light absorption of solar cells and photodetectors without affecting the surface passivation of such devices.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.